Hi I have a above ground pool with a sand filter in the UK and I'm having a real problem with algae returning to the bottom of the pool, the water balance is fine and I have added algaecide, I have vacuumed the pool very time the algae has sunk to the bottom but the next day it is back on the bottom. The algae is a green colour and the water is crystal clear.I am at my wit's end with it can anybody help me please. One other thing the Chlorine is getting used quite quickly as well.

To help the chlorine to last longer, add cyanuric acid (also called conditioner or stabilizer). Without knowing your pool volume, I can't recommend how much to add but you want to end up with a reading of about 30ppm.

As for the algae that keeps re-appearing, I would recommend backwashing your filter. Sometimes there's a condition known as 'channeling' that can effect a sand filter's ability to remove fine debris. This happens when larger 'channels' are forged through the sand bed which allow fine stuff like algae to pass right through. If you've maintained good chemistry and used an algaecide, the algae may be dead but it's now the filter's job to remove it.